The present invention relates to an oil pressure control means for an automatic transmission for vehicles, and, more particularly, to an improvement thereof with regard to downshift performance.
In an automatic transmission for vehicles comprising a fluid torque converter and a speed shift gear means having a plurality of friction engaging means and adapted to establish various speed shift stages, engagement and disengagement of said friction engaging means are automatically changed over depending upon operating conditions of the vehicle so as to establish the most desirable speed shift stage for the current operating condition of the vehicle, instant by instant. This changing-over control of the friction engaging means is generally effected by an oil pressure control means which comprises a plurality of speed shift valves, adapted to be shifted depending upon a balance of the throttle pressure which varies in accordance with the stepping-on amount of the accelerator pedal or engine throttle opening and the governor pressure which varies in accordance with the vehicle speed, thereby selecting the most desirable speed shift stage of the speed shift gear means. Furthermore, the oil pressure control means includes a manual shift valve adapted to be operated by hand by the driver for setting a particular speed shift range. The speed shift ranges in usual automatic transmissions include D range, 2 range, and L range for forward driving. Assuming that an automatic transmission is designed to provide a 1st speed stage, a 2nd speed stage, a 3rd speed stage and an overdrive stage, which are automatically selected in accordance with a balance of the vehicle speed and the stepping-on amount of the accelerator pedal, when the manual shift valve is shifted to the D range, an automatic selection of speed stage is made out of the aforementioned four speed stages. When the manual shift valve is shifted to the 2 range, speed stage selection is made from the 1st speed stage and the 2nd speed stage. When the manual shift valve is shifted to the L range, the automatic transmission is fixed in the 1st speed stage.
Now, let us assume that an automatic transmission includes overdrive means and a vehicle having such an automatic transmission is being driven in overdrive stage. Under this running condition, if the manual shift valve is abruptly shifted from D range to 2 range, the automatic transmission is abruptly shifted down by two stages from the overdrive stage to the 2nd speed stage, skipping over the 3rd speed stage. Such an abrupt two-stage downshifting is not desirable because it causes heavy frictional force to be exerted on the friction engaging elements of the friction engaging means, thereby unduly wearing these elements, while it further produces a danger of causing an over-revving of the engine. Furthermore, such an abrupt two-stage downshifting causes abrupt deceleration of the vehicle, which is not desirable in view of driving safety.